The second most common method of analyzing
crystalline silica is based on the absorption of infrared light. Compared to XRD, IR is a
less expensive method choice when interferences are known and can be compensated for. The
IR method is used by the Mine Safety and Health Administration (MSHA) for Coal Mining
samples.

The tetrahedral SiO2 structural units absorb light frequencies at
characteristic vibrations. One isolated tetrahedral unit is illustrated to the right. Note
that though each silicon atom is surrounded by four oxygen atoms, each oxygen is also
associated with other silicon atoms so that the ratio of two oxygen atoms for each silicon
atom is maintained in the crystal structure.